Machines that are powered by an engine, such as motor vehicles, typically include a transmission to adjust the rotational speed and torque between the engine and the machine. Some engines, such as internal combustion engines, typically operate at higher rotational speeds (except when operating in an overdrive condition, for example) than are desired as output for their associated machine. On the other hand, some engines, such as electric motors, may typically operate at lower rotational speeds than are desired for their associated machine. However, various types of engines will have characteristic operating range(s) that is/are considered most desirable in terms of operational speed, torque, power output, and for the mechanical health and longevity of the engine. Accordingly, a mechanical transmission is typically provided between an engine and its associated machine so that a drive ratio between the engine output and the machine output can be varied over a desired operating range of the engine. Such transmissions typically include a power train of multiple gears of varying diameters and gear ratios that can be shifted between different gear combinations, to provide a desired output rotational speed for the machine that varies from the engine operating speed, in various power and torque conditions. This allows the machine be operated closer to its desired operating torque and speed range while permitting the output to vary over different and usually broader torque and speed ranges.
In automobiles, for example, manual transmissions were developed to allow a user to manually select one of several discrete gear ratios. Automatic transmissions were later developed in which an appropriate gear ratio for given conditions and power demand are automatically determined and implemented. Conventional transmissions, whether manual or automatic, are often complicated, heavy, and bulky, and therefore expensive. Further, such systems often shift abruptly in a stepped manner between discrete ratios, rather than in a smooth and continuous manner. These characteristics of conventional transmissions tend to reduce the overall efficiency of the machine, and also introduce operational characteristics that are considered undesirable.
These characteristics become particularly noticeable in other transmission applications. For example, farm equipment typically operates within a relatively narrow range of speeds. However, a tractor, for example, may have another piece of farm equipment connected to a power take-off (PTO). The additional piece of farm equipment may be preferably operated at a nearly constant operational speed, thus involving a relatively large number of gear ratios to drive the tractor at varying speeds while maintaining the engine at a nearly constant rotational speed for the sake of the PTO.
To address some of these issues related to mechanical transmissions, continuously variable transmissions have been developed. Continuously variable transmissions select and provide output power along a continuously variable range of gear ratios, rather than in discrete steps, thus allowing more optimum and continuous operation of the engine. However, conventional continuously variable transmissions typically employ belt and pulley systems or frictional cones and the like, which rely upon friction for operation. They can also be relatively mechanically complicated. Consequently, known continuously variable transmissions tend to present significant mechanical losses, which reduces their efficiency, and are also limited in the maximum torque which they can transfer, thus limiting their use to relatively low torque applications (e.g. small motor vehicles). Other concerns also exist with conventional manual, automatic and continuously variable transmissions.
The present application is directed to overcoming one or more of the above-mentioned issues.